Several races in these mods have unusual or “non-standard” body types. For example, the Xeno Hive mod gives its insectoid species four arms, each with (from what I can tell) four fingers. Because of this, many custom races end up being incompatible, since the mod only supports the game’s default body part system and doesn’t account for the extra limbs or unique anatomy.

Plus there is no way to assign extra body parts in the mod (other then the defaults)